The giant kelp Macrocystis pyrifera is harvested commercially
from Southern California (Barilotti & Zertuche-Gonzalez, 1990) and Southern Chile
(Santelices & Ojeda, 1984a, 1984b). M. pyrifera is a remarkable seaweed because
of the large size and rapid growth rate of the sporophyte generation. Individual plants
may measure up to 50 m in length. The stipes arise from perennial, branched holdfasts
which may each cover a large area of bedrock and each stipe can bear up to 200 fronds.
Individual fronds may grow at rates of up to 7 cm per day. Reproduction is from
sporophylls around the base of the plant.

California

Information on harvesting methods from the Pacific Ocean waters of Baja
California and California is given in Barilotti & Zertuche-Gonzalez (1990). These
methods (North, 1987) are highly mechanised and utilise large boats capable of cutting and
lifting 300 to 550 t wet weight in a single load. Harvesting removes only the fronds in
the surface canopy to a depth of 1.2 m (operating on the lawnmower principal). This method
does not cut the sporophylls located around the base of the plant that provide spores for
the next generation, or the meristems that produce the new fronds. The individual fronds
have a lifespan of about 6 months (North, 1987). Estimates of biomass removed by
harvesting range from 33% to 50% of the total biomass of the plant (Coon, 1987; North,
1987). Such a large amount can be removed from these long plants when cutting at only a
shallow depth because most of the mature frond material floats on the water surface,
buoyed up by bladders.

North (1968) concluded that little damage is caused to the kelp by
harvesting and that in some circumstances harvesting may be beneficial to the plants.
These findings were largely confirmed by Coon (1987), who showed that yields and canopy
areas in Santa Barbara County, California, remained relatively constant over a five-year
harvesting period. Dayton et al. (1984) pointed out that the stability of the kelp
populations in M. pyrifera beds at Point Loma had not been noticeably affected by
harvesting. Aerial photographs of Carmel Bay, California from 1971 to 1977 (Barilotti et
al., 1985) did not indicate long-term changes in the area of the kelp beds.

Effects of harvesting on the production and recruitment of juvenile
plants have not been studied (Barilotti & Zertuche-Gonzalez, 1990). Though removal of
75% of the canopy was shown to reduce the production of sporophylls (Reed & Foster,
1984; Reed, 1987), it is expected that removal of the canopy will aid recruitment of young
plants by increasing the light available, as sporeling growth is thought to be
light-limited (Deysher & Dean, 1986).

Chile

Santelices & Ojeda (1984a) found that removal of the canopy
resulted in juvenile recruitment in Chilean M. pyrifera forests being increased,
but there was a significant decrease in interplant distances (a population density
increase) showing the change in the population structure of the M. pyrifera forest.
Removal of the canopy of Chilean M. pyrifera forests also resulted in a decrease in
the biomass of the understorey kelp, Lessonia flaviscens (Santelices & Ojeda,
1984b) though the total number of species in the experimental area remained the same.

Effects of harvesting on other species in M. pyrifera
beds

California

Bodkin (1988) found that the abundance of seven species of fish was
significantly decreased after removal of the M. pyrifera forest in southern
California. Standing stocks of fish are larger in areas with M. pyrifera than
without, in California, (Larson & DeMartini, 1984) but the abundance of fish was not
affected by a decrease in kelp abundance in warmer climates (Stephens et al.,
1984).

Grazers such as the sea urchin, S. droebachiensis, may modify
the pattern of recolonisation (Dayton et al., 1992). Increases in the populations
of grazer species may result in the development of "barrens" when the developing
and juvenile kelps are removed (Leighton, 1971). This is particularly obvious when large
numbers of grazers are congregated in a limited area, such as rocky outcrops from a sandy
substratum. These barrens can become a relatively stable community but the kelp forests
may recover if the urchins are removed by disease (Elner & Vadas, 1990) or by storm
events (Ebling et al., 1985). North & Pearse (1970) associated a population
explosion of sea urchins in Southern California with the onset of kelp harvesting, but
they also point to other possible causes: the destruction of sea otters populations which
prey on the urchins; the depletion of abalone populations which graze competitively with
the urchins; enrichment of the coastal waters with partially treated sewage (which may be
used as an amino acid source by the urchins).

Effects on kelp of the harvesting of sea urchins

Artificial removal of the red urchin, Strongylocentrotus
franciscanus, permitted a rapid increase in the biomass of foliose annual algae and
the eventual dominance of near-shore perennials (kelps etc.) in laminarian forests (Pace,
1980). M. pyrifera communities may therefore change if present densities of urchins
are reduced through commercial harvesting or the introduction of a predator such as the
sea otter, Enhydrys lutris (Pace, 1980). Estes et al. (1982) actually
associated the destructive levels of urchin grazers with the absence of sea otters. North
(1980) showed that the coralline barrens could be re-colonised by dispersing embryonic
M. pyrifera sporophytes grown in the laboratory.